The invention relates to an electroacoustic apparatus for the reproduction of lowest to medium frequencies and more specifically to an electroacoustic apparatus comprising a folded horn in combination with a loudspeaker, which folded horn is well approximated in its course to an ideal exponential curve.
The newest developments in CD technology, synthesizers and drum-computers require from the electroacoustic transducer technique the reproduction of the lowest or deepest sounds with relatively high sound pressure (e.g. for the church organ sounds of synthesizers or, as a specific example, the low beats of the drum in Bruckner symphonies on compact discs).
In order to achieve the reproduction of lowest to medium frequencies by an electroacoustic apparatus those apparatus have been developed having a housing wherein a cone-shaped loudspeaker combined with a folded horn is accommodated. The folding makes that the length as well as the mouth opening of the horn become large to increase the characteristic acoustic impedance to ensure, on the one hand, the transmission of the bass frequencies with high efficiency. On the other hand, the dimensions of the housing shall be limited such that an easy transport and the desired shape of a rectangular parallelepiped become possible
Furthermore, it is known to give the horn an approximately exponential shape in order to increase the characteristic radiation impedance at frequencies in the 100 Hertz range, i.e. a shape where the cross section of the horn follows an exponential function
When the horns are folded there are, however, a number of difficulties to achieve said approximation. Approximations have heretofore been usual by a three-stage polygonal course which resulted, however, only in an unsatisfactory approximation to the exponential function. Moreover, the folds impair the acoustic radiation characteristics due to undesired reflections.
In the horns approximated by a three-stage polygonal course three plane horn element walls are provided behind a cone-shaped loudspeaker in the upper part of the housing, which walls end in a horn throat. A bent horn element wall secured in the lower part of the housing ends in the horn mouth and defines a well-formed fold in contrast to the fold which is formed by said plane walls with the walls of the housing. A satisfactory reproduction in the range of low frequencies can, however, not be achieved with such an apparatus.